With the monthly events rewards currently being reworked, I have a suggestion. There should be a 1-2 hero roster size increase as an additional reward for completing the legendary portion of the monthly event.
With the introduction of new heroes, the different tourney rules which find a use for nearly any hero (3* included), as well as the upcoming costumes, it is getting very hard to manage rosters. The cost for new spots is also getting prohibitively expensive (up to 200 gems per increase now).
I am level 48, and the additional +2 spots every few levels is not sufficient to keep up with the heroes needed. Allowing an extra 12-24 slots over the course of a year as a reward would help out a lot.